Quote Originally Posted by QuantumEleven
You've been watching too much Lord of the Rings :) As far as I understand it, you can't mount much in the way of siege equipment on top of walls because the lateral forces they cause are too large and would cause the wall to crack.

Of course, it would be damn cool
In 1546(I think) Constinanople was beseiged by the turks.Constinanople was at that time the most heavily defended city in all time. The byzatines mounted cannons on the towers...But they didnt work because the shock shook the masonry apart.